One class of nonvolatile memory devices includes electrically erasable programmable read only memory (EEPROM), which may be used in many applications including embedded applications and mass storage applications. In typical embedded applications, an EEPROM device may be used to provide code storage in personal computers or mobile phones, for example, where fast random access read times may be required. Typical mass storage applications include memory card applications requiring high capacity and low cost.
One category of EEPROM devices includes NAND-type flash memories, which can provide a low cost and high capacity alternative to other forms of nonvolatile memory. FIG. 1A illustrates a conventional flash memory array 1 having a plurality of NAND-type strings therein. Each of these NAND-type strings includes a plurality of EEPROM cells, which are associated with respective even and odd bit lines (BL0_e, BL0_o . . . , BLn_e, BLn_o). These bit lines are connected to a page buffer 2 having a plurality of buffer circuits (PB0, . . . , PBn) therein. Each EEPROM cell includes a charge trap layer (or floating gate electrode) and a control gate electrode, which is electrically connected to a respective word line (WL0, WL1, . . . , WLn). Access to each NAND string is enabled by driving a string select line (SSL) associated with string selection transistors to a logic 1 voltage during reading and programming operations. Each NAND string also includes a respective ground selection transistor, which is electrically connected to a ground select line (GSL).
As illustrated by FIG. 1B, the EEPROM cells within the flash memory array 1 of FIG. 1A may be cells that support a single programmed state. EEPROM cells that support only a single programmed state are typically referred to as single level cells (SLC). In particular, an SLC may support an erased state, which may be treated as a logic 1 storage value, and a programmed state, which may be treated as a logic 0 storage value. The SLC may have a negative threshold voltage (Vth) when erased (e.g., −3V<Vth<−1V) and a positive threshold voltage when programmed (e.g., 1V<Vth<3V). This programmed state may be achieved by setting the bit line BL to a logic 0 value (e.g., 0 Volts), applying a program voltage (Vpgm) to a selected EEPROM cell and applying a pass voltage (Vpass) to the unselected EEPROM cells within a string, as illustrated by FIG. 1C. In addition, during programming the NAND string may be enabled by applying a positive voltage (e.g., power supply voltage Vdd) to the string select line (SSL) and a ground voltage (e.g., 0 Volts) to the ground select line (GSL).
Moreover, the programmed state or erased state of an EEPROM cell may be detected by performing a read operation on a selected cell. As illustrated by FIG. 1D, a NAND string will operate to discharge a precharged bit line BL when a selected cell is in an erased state and the selected word line voltage (e.g., 0 Volts) is greater than the threshold voltage of the selected cell. However, when a selected cell is in a programmed state, the corresponding NAND string will provide an open circuit to the precharged bit line BL because the selected word line voltage (e.g., 0 Volts) is less than the threshold voltage of the selected cell and the selected cell remains “off”. Other aspects of NAND-type flash memories are disclosed in an article by Jung et al., entitled “A 3.3 Volt Single Power Supply 16-Mb Nonvolatile Virtual DRAM Using a NAND Flash Memory Technology,” IEEE Journal of Solid-State Circuits, Vol. 32, No. 11, pp. 1748-1757, November (1997), the disclosure of which is hereby incorporated herein by reference. Charge trap flash (CTF) memory cells are also disclosed in U.S. Pat. No. 7,126,185 to Kang et al. and U.S. Pat. Publication No. 2006/0171209 to Sim et al.
Additional aspects of NAND-type flash memories are disclosed in U.S. Pat. Publication No. 2007/0070699 to Lee entitled “Nonvolatile Semiconductor Memory Device Having Dummy Bit Line With Multiple Sections,” and U.S. Pat. Nos. 6,611,460 and 6,614,688. In particular, U.S. Pat. Publication No. 2007/0070699 to Lee discloses a flash EEPROM device that sacrifices a NAND-type string of floating gate EEPROM cells in order to provide a direct connection to a common source line (CSL). This flash EEPROM device also utilizes a plurality of dummy bit segments as pocket P-well biasing straps. Moreover, the string selection transistors and ground selection transistors within each NAND-type string may be configured as disclosed at FIGS. 2, 5-6 and 10 of U.S. Pat. No. 6,881,626 Lee et al., entitled “Method of Fabricating A Non-Volatile Memory Device With a String Select Gate,” and as disclosed in U.S. Pat. No. 6,858,906 to Lee et al., the disclosures of which is hereby incorporated herein by reference.